These 3M Filtrete 4" Allergen Reduction Filters are high performance air filters designed to fit whole house media air cleaners installed in your furnace or central air conditioner. Filtrete 4" Allergen Reduction Filters contain electrostatically charged fibers, which enable these filters to attract and capture 97% of airborne allergens and particles, such as pollen, mold spores, and dust mite debris. When your furnace or central air conditioner fan is on, air circulates in your home and passes through the filter. This 3M Filtrete filter captures particles in two ways: mechanically and electrostatically. All filters perform mechanical filtration by providing a barrier that captures particles from the air passing through the filter. Filtrete filters also have electrostatically charged fibers that work like tiny magnets to attract and capture particles.